I used to encrypt passwords (PINs, CC #'s, Logins) on my Sony Clie using
Strip, a free open-source tool using AES-256bit encryption. Without much bells and whistles, but it worked (I would never trust a closed-source product in this regard).
Now, on Pocket PCs, I have yet to find a reliable
open-source implementation of a
strong cipher such as AES. Right now that leaves me totally unprotected, even my weekly backups on SD Card are unprotected (Sprite offers a password requirement, but it is easily defeated). In other words, someone could steal the card and have the full content of my PDA ;(